#include <math.h>
main()
{
     int i, count;
     printf ("There are invertable primes with 4 digits: \n");
     for ( count=0, i=1001; i<9999; i+=2)
       if ( num(i) )
         printf( count%9 ? "%3d:%d " : "%3d:%d\n", ++count, i);
}
num (int number)
{
    int j, i;
    if ( ! ok(number) ) return(0);
    for (i=number, j=0; i>0; i/=10)
     j = j * 10 + i%10;
    if ( number < j )
     if ( ! ok(j) ) return(0);
     else return(1);
    else return(0);
}
ok ( int number )
{
    int i, j;
    if (number%2==0) return(0);
    j = sqrt( (double) number) + 1;
    for (i=3; i<=j ; i+=2)
      if ( number % i == 0 ) return(0);
    return(1);
}